In such applications the antenna must be compact but it is nevertheless often required to be able to use several operating frequencies in the radio and microwave frequency bands. The frequencies can be close together and one is used to transmit and another to receive, for example. It is possible to use two frequencies because the antenna bandwidth includes both of the frequencies and all the frequencies between them. The antenna is nevertheless often required to operate in two separate bands, especially in mobile telephones. The ratio between the center frequencies of the two bands is equal to 2 in the particular case of dual-band communications systems, such as those used in the prior art GSM 900 and GSM 1800 systems, with bands at around 900 MHz and 1800 MHz.
The antennas to which the invention relates are in particular of a type referred to hereinafter as "microstrip patch antennas", in other words they have a microstrip structure in which the electric field of a traveling wave is established in a dielectric substrate between a conductive layer referred to as the "ground plane" and another conductive layer referred to as the "patch".
The operating frequencies of an antenna of the above kind are defined by one or more resonant structures included in the antenna. Broadly speaking, two basic types of resonant structure can be fabricated using the microstrip technique. A first type of structure is referred to as the "half-wave" structure. If one dimension of the patch is referred to as its length and extends in a direction referred to as the longitudinal direction, the length is substantially equal to half the wavelength of an electromagnetic wave propagating in that direction in the line consisting of the ground plane, the substrate and the patch. Coupling to radiated waves occurs at the ends of the length, which are in regions where the amplitude of the electric field in the substrate is at a maximum.
A second type of resonant structure that can be fabricated using the same technique is referred to as a "quarter-wave" structure. It differs from a half-wave structure firstly in that the length of its patch is substantially equal to one-fourth of the wavelength, the length of the patch and the wavelength being defined as above, and secondly in that a clear short-circuit is provided between the ground plane and the patch at one end of the length of the patch to impose a quarter-wave resonance with one electric field node fixed by the short-circuit. Coupling with radiated waves occurs at the other end of the length, which is in a region where the amplitude of the electric field through the substrate is at a maximum.
In practice various types of resonance can be established in such antennas, and depend in particular on:
the configuration of the patches, which in particular can incorporate slots, possibly radiating slots, PA1 the presence and location of short-circuits, and the electrical models representative of such short-circuits, which cannot always be treated, even approximately, as perfect short-circuits with zero impedance, and PA1 coupling systems included in the antennas to enable their resonant structures to be coupled to a signal processor unit such as a transmitter, and the location of such systems. PA1 a small overall size, PA1 a sufficiently large bandwidth, PA1 two separate bands, PA1 a high ratio between the center frequencies of the two bands, in particular a ratio close to 2, and in particular the facility to adjust each of the two center frequencies without significantly affecting the other one, and PA1 a low cost of fabrication. PA1 two resonant structures facing each other on respective opposite sides of a plane occupied by a conductive layer constituting a coupling layer, the two structures having respective resonant frequencies with a defined frequency ratio, and PA1 an internal coupling system including at least one slot formed in the coupling layer to enable coupling of the two resonant structures to a processor unit external to the antenna,
What is more, there can be a plurality of resonance modes for a given antenna configuration, and this enables use of the antenna at a plurality of frequencies corresponding to the resonance modes.
The invention relates more particularly to antennas referred to as "stacked" antennas in which a plurality of resonant structures are combined within the same antenna by stacking them. They then occupy different volumes.
Two antennas including, from the bottom upwards, a stack comprising a conductive ground plane, a bottom dielectric layer, a conductive layer referred to as the "coupling layer", a top dielectric layer and a top conductive layer are known in the art.
The first antenna is described in the article "Broadband stacked shorted patch", R. B. Waterhouse, Electronics Letters, Jan. 21, 1999, Vol. 35, No. 2, pp. 98, 99. It includes short-circuit conductors which greatly limit the length of each of the two stacked resonant structures.
The second antenna is described in the article "Thin dual-resonant stacked shorted patch antenna for mobile communications", J. Ollikainen, M. Fisher and P. Vainikainen, Electronics Letters, Mar. 18, 1999, Vol. 35, No. 6, pp. 437, 438. Each of its two resonant structures is of the quarter-wave type.
Each of the above two prior art antennas is fed, in other words coupled to a signal processor unit such as a transmitter or a receiver, via a coaxial line whose ground conductor and axial conductor are respectively connected to the ground plane and to the coupling layer of the antenna. Choosing the position of the point of connection between the axial conductor and the coupling layer is critical and leads to a high fabrication cost. What is more, despite the presence of two partly separate resonant structures, it appears that coupling is required between the two structures and it is not apparent that such coupling enables the structures to operate in two bands that are as far apart as is often required. In particular, it is not apparent that the ratio of the center frequencies of the two bands can easily be as high as 2.
A third prior art antenna is described in the article "Broadband CPW fed stacked patch antenna", W. S. T. Rowe and R. B. Waterhouse, Electronics Letters, Apr. 29, 1999, Vol. 35, No. 9, pp. 681-682. It includes in particular, from the bottom upwards, a ground plane including a coplanar feed line, a dielectric line, a dielectric layer, a patch, two dielectric layers, a patch and a dielectric layer. These layers form two stacked resonant structures. As in the first and second prior art antennas, coupling appears to be required between the two structures and opposes operation in two bands that are as far apart as is desirable.
Unlike the preceding structures, the resonant structures of a fourth prior art antenna are not of the patch type. The fourth antenna is described in the article "Stacked Dielectric Antenna for Multifrequency Operation", A. Sangiovanni, J. Y. Dauvignac, Ch. Pichot, Microwave & Optical Technology Letters, Vol. 18, No. 4, July 1998, pp. 303-306. It associates three resonant structures which are of a type referred to as the "dielectric type", meaning that each consists of a dielectric block with appropriate permittivity and dimensions. The overall size of the fourth prior art antenna would not appear to be as small as is often required.